1. Field of the Invention
Embodiments of the invention generally relate to optical based temperature sensors.
2. Description of the Related Art
Various approaches exist for intelligent wells that monitor temperatures within oil and gas wellbores. Some reasons for monitoring these temperatures include reducing operating costs and increasing yield from individual reservoirs. Cost effectively providing more accurate and reliable measurements over a period of time can therefore improve benefits provided by these intelligent wells.
Sensors for measuring the temperatures in the wellbore can include optical sensors, which avoid problems associated with electrically based systems. A plurality of the optical sensors can form an array of optical sensors disposed along an optical cable that includes an optical transmission waveguide such as an optical fiber. The array of sensors can include a plurality of optical Bragg gratings that each return a signal whose wavelength varies with applied temperature. These arrays can be interrogated by, for example, time division multiplexing or wavelength division multiplexing.
As another example of optical based temperature measurement that can be utilized in the wellbore, the optical waveguide itself can be employed as a distributed temperature sensor (DTS) to provide more than one measurement along its length. The DTS can be based on analysis (e.g., Raman scattering analysis) of reflected light that is altered in accordance with the temperature of the waveguide. Processing such reflections as a function of time derives temperature as a function of well depth with earlier reflections indicating the temperature at relatively shallow depths.
The signals from these sensors disposed at discrete points or from the DTS can undesirably be influenced by other parameters than temperature, thereby altering a response indicated by the signal. For example, strain or pressure applied to the sensor may dynamically change making calibration of the signal to account for the strain impossible since there is no way to tell which parameter is contributing to the response of the sensor. Accordingly, multiple parameters contributing to the response undermine accuracy and confidence in temperature measurements utilizing either DTS or Bragg grating based sensors.
Therefore, there exists a need for apparatus and methods that perform improved discrete point temperature sensing using optical waveguides.